Quantification of compounds at trace levels in the air often requires separation of a complex mixture and selecting molecules of interest for analysis. One technique used for this purpose for the separation of ions at atmospheric pressure is Ion Mobility Spectrometry (IMS), see for example G. A. Eiceman, Ion-mobility spectrometry as a fast monitor of chemical composition, Trends In Analytical Chemistry). IMS is widely used for many security applications, for example to detect explosives in airports.
Differential Mobility Analyser (DMA) technology has been used to separate electrically charged aerosol particles in the air, see e.g. Steer et al. (2014). Recently this method has been extended to the separation of ions at atmospheric pressure; see for example Martinez-Lozano and de la Mora (2006) or Santos at al. (2009).
US2005/0006578 discloses a type of DMA in the form of a cross-flow ion mobility analyser (CIMA) comprising at least two electrodes disposed so as to create an electric field therebetween, and a gas flow that opposes the electric field. Ions are carried through a channel by the gas flow and ions of a specific mobility are trapped by the opposing electric field and flow field within the channel and are detected when the ions reach the end of the channel. A detector at the end of the channel sees a continuous stream of mobility-selected ions, the ions being selected by modifying the electric field and/or the velocity of the flow field.
Another cross-flow DMA is disclosed in US 2006/0054804 A1 which provides a system for performing ion or particle mobility spectrometry. The system operates by first receiving a sample for analysis. Next, the system ionizes the sample and injects the ionized sample into a laminar gas flow. An electric field crosses the laminar gas flow so that the laminar gas flow and the electric field combine to spatially separate ions of the analytes based on ion mobility and so that the spatially separated ions contact different elements of an electrometer array. The system then analyses the output of the electrometer array to determine the mobility of the analytes.
A DMA device comprises an ionisation chamber, a separation chamber and an electric current measuring means. A sample of air containing molecules of interest is ionised in the ionisation chamber. The ions are then drawn into the separation chamber via an inlet. In the separation chamber, a linear electric field applied across the velocity flow spatially separates ions of different mobility. At the opposite electrode of the separation chamber, an outlet is positioned at some distance from the inlet. The apparatus is set up so that only ions of particular mobility can reach the outlet and progress on to an ion current measuring device which can be, for example, an electrometer based upon the Faraday cup where ions impinge on the collector and carry an electric charge so that an ion current can be measured. Variation in the electric field enables ions of different mobilities to be directed to the outlet. Thus, by measuring ion currents are various field strengths, ion mobility spectra can be recorded.
There is an increasing demand for more sensitive explosives detection technologies for a wide range of homeland security applications, particularly transport security. The low vapour pressure of some explosives presents challenges to current IMS devices. An even greater challenge is to detect concealed explosives in small quantities. However, a major problem with detecting and quantifying chemicals at very low concentrations is that, at such low concentrations, background signals from interfering chemical compounds become more prominent and this can lead to false positive signals being detected. To overcome this problem, higher resolution ion selecting means are required.
In U.S. Pat. No. 7,928,374 B2 a DMA was interfaced with an atmospheric pressure ionization mass spectrometer (APCI-MS) to improve ion identification and resolution. This improves resolution but a mass spectrometer is a large expensive device and along with a DMA the system becomes too large and expensive for many applications.
A known type of DMA apparatus is shown in FIG. 1 below. It is known that the resolving power (Rp) of a DMA of the type shown in FIG. 1 can be defined as the ratio of the sheath gas flow rate (Qsh) to the ion sample flow rate (Qi): Rp=Qsh/Qi. In a DMA of the type shown in FIG. 1, the resolution provided by the instrument is not influenced by the geometry, for example by the gap between the electrodes that create the ion-separating electric field (shown schematically in FIG. 1 below as electrodes (6) and (7)) or by the distance between the baffle (3) and the lower electrode (7). The resolving power is however influenced by the ratio of the gap between electrodes to the thickness of the bundles of ion trajectories in a sample gas flow. The ion and neutral molecule trajectories are governed by the continuous media laws of motion that lead to an expression Rp=Qsh/Qi. Thus, to reduce the thickness of the bundles of ion trajectories, it is necessary to increase the sheath gas flow rate Qsh or/and decrease the ion sample flow rate Qi.
In practice, in order to increase the resolution Rp, the sheath flow is increased. To achieve a resolving power of sufficient magnitude for practical applications, the sheath flow typically needs to be much greater than the ion sample flow, with the result that the velocity field created by the sheath gas flow in a DMA can often be close to the speed of sound. This creates two significant problems. Firstly, creating such high sheath gas flow rates requires powerful and therefore large and expensive pumps. Secondly, it leads to high Reynolds numbers and thus the flow in the DMA becomes turbulent. The turbulence has a profound effect on the resolving power by reducing it due to the formation of eddies and increasing broadening of the ion trajectories. To increase Rp by reducing the sample flow is not desirable either because it decreases the number of ions coming out of the DMA and therefore decreases the sensitivity.